1. Field of the Invention
The present invention relates to a headbox for a machine or producing a fibrous web, in particular a paper or cardboard web, from at least one fibrous stock suspension.
2. Description of the Related Art
German disclosure document DE 44 16 898 A1 describes a headbox for a paper machine including a feed device for a stock suspension comprising a guiding element with a plurality of channels; upstream from the guiding element is a mixing chamber with several infeed devices which are distributed over the headbox for a fluid which is to be added to the stock suspension and which differs in its characteristics from the stock suspension; and a nozzle chamber forming an outlet opening for the stock suspension, located immediately downstream from the guiding element. The feed devices for the fluid which is to be added extend essentially vertically in the mixing chamber and comprise several dosing openings located on top of each other.
German disclosure document DE 44 16 899 A1 describes a headbox for a paper machine including a feed device for the stock suspension, with a first guiding element comprising a plurality of channels, a second guiding element comprising a plurality of channels, an intermediate mixing chamber, a contiguous nozzle chamber forming an outlet opening for the stock suspension, and several feed lines distributed over the headbox for the fluid which is to be added. The majority of the channels of the guiding elements are positioned and dimensioned so that the outlet opening of the upstream guiding element is not aligned with a confluence into the downstream guiding element. The feed lines for the fluid which is to be added are equipped with dosing openings, the majority of which align respectively with a confluence into the downstream guiding element.
In addition, German disclosure document DE 44 16 909 A1 describes a headbox for a paper machine with a feed device for the stock suspension and a downstream contiguous guiding element equipped with a plurality of channels. Located downstream from there is a mixing chamber extending across the width of the headbox with several separation walls distributed across the headbox and extending essentially in a flow direction, as well as feed devices for a fluid which is to be added, distributed in a cross direction of the headbox and which are equipped with dosing openings. Downstream is an additional guiding element equipped with a plurality of channels and adjacent to it, one nozzle chamber forming an outlet opening for the stock suspension.
The feed devices for at least one fluid which is to be added extend into the mixing chamber and the sectional area which is formed in the mixing chamber by the separation walls begins in the region of the dosing openings from where it extends essentially in a flow direction to the second guiding element.
The headboxes described in the three documents include perforated distribution pipe plates having relatively long lengths. Due to relatively strong fluid frictions, great flow speeds and similar conditions, these long lengths cause large pressure losses in the fibrous stock suspension which are disadvantageous for the operating stability and the desired compact construction of a respective headbox. In addition, the known perforated distribution pipe plates, whether they are in the embodiment of perforated plates or tube banks, are very cost intensive in their production.
In a headbox with a feed device in the embodiment of a cross-distributor the primary function of the perforated distributor pipe plate of the headbox is to divert the fibrous stock suspension, which is delivered in the cross machine direction, to flow into the longitudinal machine direction. The perforated distributor pipe plate is essentially comparable to a “short” tube bank and it also must ensure a uniform volume flow, especially a longitudinal volume flow over the machine width, as well as effect emergence of the fibrous stock suspension flow from the distributor pipe plate to be as aligned as possible, especially longitudinally aligned.
The homogenous distribution of the volume flow of the fibrous stock suspension over the machine width is determined by the pressure loss in the perforated distribution pipe plate. In addition, acceleration of the fibrous stock suspension into the channels of the perforated distribution pipe plate should occur at a ratio of approximately ½ to ⅓. Under these conditions the tendency toward contamination and the associated stagnation point problem on the infeed side of the perforated distribution pipe plate which is known to the expert is herewith avoided.
What is needed in the art is an improved headbox that provides a trouble free operation of the headbox, even with a low pressure loss in at least one fibrous stock suspension. In addition, a process technologically sufficient operational stability across an expanded operational area with an even more compact construction should also be achieved.